SOLVENT EFFECT ON PHOTOINDUCED ELECTRON-TRANSFER BETWEEN C-60 AND 3,3',5,5'-TETRAMETHYLBENZIDINE STUDIED BY NANOSECOND LASER PHOTOLYSIS

Photoinduced electron transfer from 3,3',5,5'-tetramethyl benzidine (TMB) to C-60 in polar and non-polar solvents has been investigated by nanosecond (ns) laser photolysis in the visible and near-IR regions. The transient absorption bands of triplet C-60 (C-T(60)*), the C-60 radical anion (C-60(.-)) and the TMB radical cation (TMB(.+)) were observed in both non-polar and polar solvents. The rate constants of electron transfer from C-T(60)* to TMB, which were evaluated from the decay of C-T(60)* and growths of TMB(.+) and C-60(.-), decrease with increasing solvent polarity. In non-polar solvents, TMB(.+) and C-60(.-) seem to be produced immediately after the ns laser excitation at the same time as the formation of C-T(60)*, suggesting a weak interaction between C-60 and TMB, before electron transfer. On decreasing the solvent polarity, the kinetics of the back-electron-transfer reaction from C-60(.-) to TMB(.+) change from second-order to first-order, indicating that C-60(.-) and TMB(.+) exist as ion pairs in non-polar and less-polar solvents. The back-electron-transfer rates also tend to decrease with the solvent polarity.